An amplifier is a device that input circuit receives an input signal (e.g., current or voltage) and generates an output signal based on it; some amplifiers may amplify the power, voltage, and/or current of the input signal. A comparator is a device that compares the input signal to a reference, and outputs logic high or low accordingly. For both amplifiers and comparators, their input circuit plays a key role in determining their overall performance such as input common mode range, noise, offset, distortion, bandwidth/speed, etc.
A common type of amplifier is a differential amplifier, which receives positive and negative versions of an input signal and amplifies the difference between them. This type of amplifier is useful in noisy environments because noise that affects the positive and negative versions of the input signal equally (i.e., “common-mode” noise) does not affect their difference and is therefore not amplified. Thus, the common-mode voltage level of the differential inputs may vary within a voltage range without affecting the output of the differential amplifier.
One type of differential amplifier operates on input differential signals having a common-mode voltage anywhere between the power-supply voltage and ground (i.e., a “rail-to-rail” voltage swing). A single differential pair (of MOSFET, BJT, or other types of transistors) is insufficient to handle a rail-to-rail input common-mode range at least because the threshold voltages of the devices in the differential pair, the headroom consumed by bias transistors supporting the differential pair, and/or other factors prevent the single differential pair from remaining on and functional for the entire rail-to-rail range. Thus, differential amplifiers may use two or more complementary differential pairs in order to function properly in a rail-to-rail input common mode range; each differential pair covers part of the range. Similarly, a comparator (or other input circuit) may require complementary transistors to handle a rail-to-rail input common mode range.
For noise, stability, offset, and/or distortion reasons, it may be required that each of the two or more input differential pairs convert the input signal at the same rate so that, for example, a differential signal having a common-mode voltage near the power supply voltage is converted at the same rate as an identical differential signal having a common-mode voltage near ground. Existing circuits typically use complicated and power-hungry control circuitry to ensure that the conversion rates (also known as “transconductance,” defined as the change in output current produced by a change in input voltage) are constant, or nearly so, over the entire rail-to-rail range. These circuits are generally expensive to design, manufacture, and operate. Even under some circumstances where constant transconductance may not be absolutely required, however, a compact, efficient, power-conserving input circuit is always desirable. A need therefore exists for a simple, low-power, and robust rail-to-rail input circuit that may be used for amplifiers, comparators, or any other type of devices where input differential pair is needed.